None.
Not applicable.
This invention relates to a method of utilizing a low temperature heat source, typically less than 500xc2x0 F. (260xc2x0 C.), to drive a thermodynamic cycle which can be used to either heat or cool an environmentally controlled space, such as a building.
Many industrial processes produce waste heat of low temperature, such that little useful work is generally accomplished with this waste heat. It is well known that certain thermodynamic cycles, such as absorption refrigeration, can provide environmental cooling even from low grade heat sources, such as thermal solar, gas turbine engine exhaust, and bottoming cycles for industrial steam generators, but absorption cooling suffers from low efficiencies. In addition, some processes, such as thermal solar, cannot generate sufficiently high temperatures to drive typical thermodynamic cycles, without expensive concentrating and collecting systems, thus requiring the system coefficient of performance to be as high as possible, in order to minimize collector cost.
Prior art has produced various combinations of thermodynamic cycles providing heating andor cooling. None have maximized the efficiency achievable with such a cycle. Some systems pass all fluid through a gas compressor, requiring substantially more power than pressurizing a liquid (Katzow, 2,511,716). Other approaches either do not regenerate heat from the working fluid (Steuart, 1,871,244) or do not regenerate heat in a fashion that maximizes the temperature of the working fluid entering the heating device (Brola, 4,118,934). Some systems do not absorb heat from the outside atmosphere as part of the heating cycle (Brola, 4,118,934), limiting the coefficient of performance to one or less. Some systems attempt to only provide heating (Schafer, 4,271,679) or cooling (Horn, 2,875,589) but not both. Some add complexity by using separate working fluids for the power and heat pump cycles (Silvern, 3,153,442) (Schafer, 4,271,679). None provide a simple, yet flexible, method of control, such that the system can be controlled over a wide range of heat input energy.
Hence, there is a need for a single system of sufficient efficiency and simplicity to make the manufacture and operation economically attractive. Since the intent of the system is to operate off waste heat, the heating and cooling system must be flexible enough to accommodate variable heat energy input and allow simple controls with a minimum of sensors and actuators.
The present invention provides a process and apparatus for utilizing waste heat to power a reconfigurable thermodynamic cycle that can be used to selectively cool or heat an environmentally controlled space, such as a room or a building. The present invention also provides a method of controlling the system, while allowing large variations in the heat input rate. The system provides a design which reasonably balances the need to maximize efficiency, while also keeping the design cost effective.
The thermodynamic cycle of this invention is a combination of a Rankine cycle to provide power and a refrigeration and heating cycle, conmmonly known as a heat pump. When in the heating mode, coefficients of performance greater than one can be achieved by this system. This means that this process will transfer more heat to the desired environmentally controlled space, than if the waste heat were used directly to heat the same space.
The system uses a single working fluid in both liquid and gaseous phases. The advantage of a single working fluid is that it eliminates the need for dynamic seals between the rotating mechanical components, thus allowing a hermetically sealed system similar to most air conditioning systems in current use. The working fluid has desirable properties of low critical point pressure ( less than 1000 psia) and temperature ( less than 300xc2x0 F.). Several common refrigerants are candidate working fluids.
While not a requirement to make the system function, using supercritical conditions in the Rankine cycle can have a significant impact on the overall system efficiency. This effect is recognized when looking at the pressure-enthalpy and temperature-entropy characteristics of several refrigerants. At highly superheated temperatures, greater amounts of work can be extracted from the fluid for a given pressure ratio than at lower temperatures. Operating at supercritical pressures allows the greatest pressure drop across the prime mover as possible. But operating at these conditions leaves an excessive amount of energy remaining in the fluid being exhausted from the prime mover. Thus to maximize efficiency, the excess energy must be recovered and used to preheat the working fluid, commonly called regeneration. Operating at supercritical conditions and regeneration are the two key elements to maximizing the system coefficient of performance.
In one embodiment of the present invention, and by way of example only, the environmental cooling is provided as follows: The liquid refrigerant is pressurized to supercritical pressure, a heater raises the fluid temperature to superheated conditions, the fluid is passed through a prime mover of the gas expander type, and the expander exhaust is flowed through the first of two regenerators to preheat the liquid refrigerant prior to entering the heater. The expander is mechanically coupled to a compressor which pressurizes the refrigerant in vapor phase from a low pressure vapor to a vapor at the same pressure as the expander outlet. The outlet of the first regenerator and the compressor outlet are combined and passed through a second regenerator prior to the combined fluid entering a condenser. Within the condenser, the vapor is cooled to a liquid phase. A portion of the condensed liquid is passed through an expansion valve and through an evaporator, where the building air is cooled. The vaporized low pressure refrigerant is then returned to the inlet of the compressor. The remainder of the liquid refrigerant that did not pass through the expansion valve is returned to the high pressure pump to be once again pressurized, preheated by the two regenerators, and returned to the heater.
The present invention also develops a method of control for the heating and cooling apparatus, allowing for required system flexibility with a minimum of sensors and actuators. Because the heat input may be variable and uncontrolled, the system controls must have the flexibility to accommodate any set of heating temperatures and heat input rates. This means the load on the system must be controlled to match the input power. Plus, based on prior discussions on maximizing pressure and temperature to achieve maximum work out of the prime mover, control laws are presented which can be used to optimize the power cycle operating point based on input power and achievable working fluid temperatures.
Environmental heating is accomplished in an almost identical fashion, with the exception that the working fluid is vaporized by exposing the evaporator to ambient atmospheric conditions and heat is rejected to the environmentally controlled space from the condenser.
Accordingly, besides the objects and advantages of the cooling and heating apparatus described above, several objects and advantages of the present invention are:
(a) to provide cooling and heating apparatus and process which is powered by heat energy from low temperature, waste heat sources, such as thermal solar, gas turbine engine exhaust, residual energy from steam generators, or any one of many other similar sources.
(b) to provide a cooling and heating system which can accommodate variable amounts of input energy and wide variation of temperature of that input energy.
(c) to provide maximum efficiency in both cooling and heating modes for a system which is a combined cooling and heating system and which utilizes waste heat.
(d) to provide a means for achieving maximum temperature of the pre-heated working fluid, prior to entering the primary heater.
(e) to provide a cooling and heating system which is simple in design and, therefore, is more cost effective to manufacture.
(f) to provide a system which is easy to control, requiring a minimum number of sensors and actuators.
(g) to provide a system which requires a minimum of actuation devices to transform the system from cooling mode to heating mode and vice-versa.
(h) to provide, for a waste-heat driven cooling and heating system, a control algorithm which can operate the system at near optimum performance while accommodating a wide range of operating conditions and continuously varying operating conditions, including start-up and shut-down transients.
The novel features which are believed to be characteristic of the invention, both as to its organization, method of operation, and control, together with further objects and advantages thereof will be better understood from the following description, when considered in connection with the accompanying drawings in which a presently preferred embodiment of the invention is illustrated by way of example. It is expressly understood, however, that the drawings are for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.